This invention relates generally to a clinical method for medical diagnosis and, more particularly, to an improved method for the preparation of blood sample films or "smears" on microscope slides or similar substrates. While applicable to all techniques for evaluation of the resulting smears, the method and apparatus are especially adapted for use in connection with automated blood cell analysis using computerized pattern recognition systems.
The traditional method of preparing blood smears on microscope slides for evaluation by physicians or medical technicians consists of placing a small quantity of the blood specimen on a microscope slide and then manually wiping the blood across the slide with the edge of another slide or a cover glass. While reasonably satisfactory for the preparation of smears for microscopic examination by skilled technicians or physicians, the obvious variations in results from slide to slide inherent in such a manual technique obviously renders it wholly unsuitable to preparation of slides for evaluation by automated pattern recognition apparatus.
An early attempt to achieve a high degree of control and uniformity in the preparation of blood smears is described in U.S. Pat. No. 3,577,267 granted to K. Preston, Jr. et al. on May 4, 1971. The method and apparatus proposed by Preston et al. involves placing a small specimen on a substrate and then spinning the substrate at high speeds, thus utilizing centrifugal force to form an evenly distributed thin layer of blood on the substrate surface with a minimum of disturbance to blood cell morphology. This involves rapid acceleration of the substrate and spinning at high speeds in the range of from 4,000 to 10,000 rpm according to the Preston et al patent. Consequently, the greater portion of the original specimen is spun off the slide, much of it in the form of aerosol. A shield is provided around the spinner to prevent widespread scattering of the spun-off blood but does little or nothing to prevent aerosol from entering the atmosphere of the laboratory.
Inasmuch as some of the blood specimens routinely handled in hospital and private clinical laboratories contain pathogenic organisms, the use of spinners and the resultant dispersal of aerosol constitutes a health hazard for clinical personnel, exposure to hepatitis virus being a particular problem.
The containment of aerosol by disposing the spinner in an enclosed chamber such as that shown in U.S. Pat. No. 3,705,048 to J.J.J. Staunton, represents a partial solution only because the aerosol persists within the chamber and eventually escapes into the atmosphere when the cover is opened.
Another problem encountered in the utilization of centrifuges to prepare blood smears is the variation in hematocrit from specimen to specimen which makes it necessary to adjust the spinning parameters (e.g., acceleration, speed and spinning time) in order to obtain a degree of uniformity in the resulting smear. In this connection, reference may be had to U.S. Pat. No. 3,906,890 to Amos et al. describing a blood smear centrifuge in which spinning time is automatically adjusted in accordance with the hematocrit of the specimen. Obviously, it would be preferable if the spinning parameters could be the same for all or at least the vast majority of blood specimens and, to this end, it has been suggested (J. W. Bacus, Erythrocyte Morphology and Centrifugal "Spinner" Blood Film Preparations, J. of Histochemistry and Cytochemistry, Vol. 22, No. 7, pp. 506-516) that it would be desirable to adjust the hematocrit in order to permit slides to be spun at the same speed and for the same time. Adjustment of the hematocrit may be accomplished by dilution of the blood specimen with physiological saline. To avoid the necessity of adding sample dilution to the procedures performed by the operator and to ensure uniformity in the proportions of sample and diluent, it is highly desirable that the dilution be carried out automatically. It is to the automatic accomplishment of dilution as well as the prevention of atmospheric contamination with aerosol that the present invention is addressed. It is the basic general object of the invention to provide a blood smear preparation method which overcomes or mitigates problems of the prior art as outlined hereinabove.
A more specific object is the provision of a sample preparation method for preparing blood smears on substrates which are particularly suitable for evaluation by computer operated pattern recognition apparatus or the like which produces smears of high uniformity and reproducibility.
A further object is the provision of a method in accordance with the preceding objects which effects blood specimen dilution and spinning essentially without operator intervention and which effects the formation of a blood smear by centrifugal action without the attendant hazard of dispersing possibly pathogenic blood particles and aerosol into the atmosphere.